In the oil and gas industry, seismic exploration techniques are commonly used to aid in locating subsurface deposits of oil, gas, and other useful minerals. Because drilling involves high costs and high risks, seismic surveys are used to produce an image of subsurface geological structures. While the image may not directly show the location of oil or gas, those trained in the field can use such images to more accurately predict the location of oil and gas and thus reduce the chance of drilling a non-productive well.
Seismic exploration, whether on land or at sea, is a method of detecting geologic structures below the surface of the earth by analyzing seismic energy that has interacted with the geologic structures. Generally, a seismic energy source imparts a force at the surface of the earth. The resulting mechanical stress propagates according to the elastic properties of the subsurface, and is at least partially reflected by subsurface seismic reflectors (interfaces between geologic structures that have different acoustic impedances). Seismic receivers, placed at or near the earth's surface, within bodies of water, or below the earth's surface in wellbores, record the ground motion or fluid pressure resulting from the reflection. The recordings are processed to generate information about the location and physical properties of the subsurface geologic structures that reflected the seismic energy.
Various types of sources have been employed to impart seismic energy into the earth, but most fall into one of two general categories: impulsive or vibratory. An impulsive source, such as an explosive or airgun, generates a short, high-amplitude force, injecting a large amount of energy into the ground in a brief time. Recordings generated using impulsive sources generally have a high signal-to-noise ratio, which facilitates subsequent processing. However, the use of impulsive sources can present safety or environmental concerns.
By contrast, a vibratory seismic energy source generates a lower-amplitude force over a longer period of time. The resulting recordings generally have a lower signal-to-noise ratio than those generated with impulsive sources. Furthermore, because the imparted force typically extends over a time much longer than the interval between reflections, the recorded data generally contains multiple overlapping reflections. However, subsequent processing can correlate the recorded data with the imparted force to approximate the data that would have been collected using an impulsive source.
Vibratory sources also permit some control over the characteristics of the imparted force. For example, to facilitate data collection, subsequent data processing, or both, it is often desirable to impart a force with energy at one or more desired frequencies, and to vary those frequencies over time. Such a controlled force is typically referred to as a “sweep.” The difference between the highest and lowest frequencies contained in the sweep is known as the “frequency range” of the sweep, and the length of time over which the source generates the sweep is known as the “sweep time.” Many different forms of sweep may be useful in a seismic survey. For example, a sweep may include a single sinusoid at a fundamental frequency that starts low and varies monotonically upward (an “upsweep”) or a fundamental frequency that starts high and varies monotonically downward (a “downsweep”). Such sweeps may be linear, with the fundamental frequency changing at a fixed rate over the entire sweep time. Sweeps may also be non-linear, for example a quadratic or logarithmic sweep. Alternatively, a sweep may include an unvarying fundamental frequency, a mixture of multiple frequencies, an exotic signal such as a pseudo-random sequence, or any other desired signal.
Vibratory sources can take a number of different forms. For example, recent land surveys have often employed servo-controlled hydraulic “shaker units” mounted on trucks. Marine sources typically include a towed bell-shaped housing, with a diaphragm in its open end vibrated by a hydraulic system similar to the land-based shaker units. However, alternative designs have been used, and the term “vibratory source” is intended to encompass any seismic vibrator, whether used on dry land or at sea.
Land-based seismic vibrators typically include several components all carried on a single vehicle. The vehicle may be referred to as a “vibrator truck.” The components of the seismic vibrator include a baseplate which is held in contact with the ground, a reaction mass that can move up and down for P-wave vibrators or move back and forth horizontally for S-wave vibrators, a high-pressure hydraulic system that can apply a force between the baseplate and the reaction mass to generate vibrations, and a control system.
These components may impose constraints on the maximum amplitude of the seismic signal that the seismic vibrator can generate at a particular frequency. For example, if the hydraulic system applies too great a force to the reaction mass, the baseplate may lose contact with the ground. As another example, the reaction mass has a limited range of displacement from its neutral position. These constraints correspondingly limit the energy contained in the sweep. As a result, the sweep must be adapted to the constraints imposed by the components of the seismic vibrator.
A sweep containing a single fundamental frequency at a given time may make inefficient use of the vibratory seismic source. For example, a vibratory seismic source may be limited, at low frequencies, by the maximum displacement of the reaction mass. However, at those low frequencies, the force applied by the hydraulic system may be substantially less than the maximum force available. By contrast, at high frequencies, the source may be limited by the maximum force, while the displacement of the reaction mass is significantly smaller than the maximum.
Accordingly, there is a need for systems and methods that can increase the energy contained within a sweep, while still being adapted to the constraints imposed by the components of the seismic vibrator.